The present invention relates to hexagonal plate-like barium ferrite particles for use in magnetic recording, obtained by substituting a part of Fe(III) in plate-like barium ferrite particles with Ti(IV) and Co(II) or Co(II) and a divalent metal other than Co such as Fe(II), Mn(II) and Zn(II), and a process for producing the same.
In more detail, the present invention relates to hexagonal plate-like barium ferrite particles for magnetic recording, represented by the formula: BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 wherein x and y are independently 0.6 to 1.2 and M is Co(II) or Co(II) and a divalent metal other than Co, having the width of the particle of 0.1 to 0.4 micrometer and a BET specific surface area (S) of 20 to 50 m.sup.2 /g, and showing a magnetization (M) of larger than 40 emu/g in a magnetic field of 10 KOe and a coercive force (Hc) of 500 to 1000 Oe, and a process for producing hexagonal plate-like barium ferrite particles for magnetic recording, represented by the formula: BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 wherein x and y are independently 0.6 to 1.2 and M is Co(II) or Co(II) and a divalent metal other than Co, which comprises autoclaving an aqueous highly alkaline suspension of ferric hydroxide containing Ti(IV), Ba(II) and Co(II) or Co(II) and the divalent metal other than Co as M(II) at a temperature in the range of 250.degree. to 330.degree. C., treating the thus obtained particles with an aqueous solution containing Si(IV), and subjecting the thus treated particles to thermal treatment at a temperature of not more than 900.degree. C.
In recent years, accompanying with the propagation of VTR, audio component, word processer and computer, non-acicular ferromagnetic particles of a favorable dispersibility provided with a suitable coercive force (Hc) and a large magnetization (M) have been required as the magnetic material for recording, particularly as the magnetic material for the perpendicular magnetic recording.
In general, as a method for producing barium ferrite particles, a method of heat-treating an aqueous alkaline suspension containing Ba(II) and Fe(III) in an autoclave as a reactor (the method is hereinafter referred to as "the autoclaving method") has been known, and by selecting the reacticn conditions in the autoclaving method, barium ferrite particles precipitate. The thus precipitated particles are hexagonal plate-like, and the distribution of the particle size and the average size of the particles differ according to the reaction conditions. In result, the particles differ in their properties.
The present inventors found that hexagonal plate-like barium ferrite particles represented by the formula: BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 wherein x and y are independently 0.6 to 1.2 and M is Co(II) or Co(II) and a divalent metal other than Co such as Zn(II), Mn(II) and Fe(II), were produced by autoclaving an aqueous highly alkaline suspension of ferric hydroxide containing Ti(IV), Ba(II) and as M(II), Co(II) or Co(II) and the divalent metal other than Co such as Fe(II), Zn(II) and Mn(II).
However, in the case where M(II) is Co(II), in the obtained hexagonal plate-like barium ferrite particles represented by the formula: BaFe.sub.12-x-y Co.sub.x Ti.sub.y O.sub.19 wherein x and y are as defined above, x becomes larger, the coercive force (Hc) thereof becomes smaller. The coercive force (Hc) of the hexagonal plate-like barium ferrite particles is 100 Oe, when x is 1.2. Also, a magnetization (M) of the hexagonal plate-like barium ferrite particles in a magnetic field of 10 KOe is less than 40 emu/g, when x is larger than 0.3.
In the case where M(II) denotes Co(II) and divalent metal such as Fe(II), Zn(II) and Mn(II), a magnetization (M) of the hexagonal plate-like barium ferrite particles in a magnetic field of 10 KOe slightly changes by a kind and amount of the divalent metal.
As a result of the present inventors' study for improving the magnetization (M) in a magnetic field of 10 KOe of the hexagonal plate-like barium ferrite particles represented by the by the formula: BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 wherein x,y and m are as defined above, it has been found that the thermal treatment of the particles obtained by autoclaving method formed a hexagonal plate-like barium ferrite particles showing the high magnetization (M) without remarkably changing the shape of the particle as well as a BET specific surface area (S), and based on the finding, the present invention has been attained.